Precision variable resistor for high-frequency use

ABSTRACT

A precision variable high resistor for high-frequency use comprises a complex series condenser composed of three spaced, parallel plates. The intermediate plate is mechanically movable relative to the pair of outer plates in parallel relation thereto. The three plates comprise discs of dielectric material having thin metal electrodes attached to their facing surfaces; and the electrodes attached to their facing surfaces; and the electrode which is attached to one of said outer plates, and which faces said intermediate plate, is grounded. A fixed resistance is connected between said grounded electrode and the facing electrode on said intermediate plate.

United States Patent [72] Inventor Bunjiro Ichijo 2-23-10 Hirosawa, Hamamatsu, Japan 21 Appl. No. 62,835 [22] Filed Aug. 11, 1970 [45] Patented Dec. 21, 1971 [32] Priorities June 16, 1970 [33] Japan [31] 45/52099;

Aug. 15, 1969, Japan, No. 44/64617 [54] PRECISION VARIABLE RESISTOR FOR HIGH- FREQUENCY USE 3 Claims, 12 Drawing Figs.

[52] U.S. Cl 324/62 R, 333/81, 338/60 [5]] G0lr 27/02 [50] Field of Search 333/81; 324/62, 63, 57, 57 Q [56] References Cited UNITED STATES PATENTS 2,337,759 12/1943 Loughlin 324/57 Q UX Primary Examiner-Herman Karl Saalbach Assistant Examiner-Marvin Nussbaum Attorneys-William D. Hall, Elliot I. Pollock, Fred C. Philpitt, George Vande Sande, Charles F. Steininger and Robert R. Priddy ABSTRACT: A precision variable high resistor for highfrequency use comprises a complex series condenser composed of three spaced, parallel plates. The intermediate plate is mechanically movable relative to the pair of outer plates in parallel relation thereto. The three plates comprise discs of dielectric material having thin metal electrodes attached to their facing surfaces; and the electrodes attached to their facing surfaces; and the electrode which is attached to one of said outer plates, and which faces said intermediate plate, is grounded. A fixed resistance is connected between said grounded electrode and the facing electrode on said intermediate plate.

PRECISION VARIABLE RESISTOR FOR HIGH- FREQUENCY USE BACKGROUND OF THE INVENTION Recently, in the measurement of a dielectric loss angle in a high frequency region, it has become necessary to measure very small dielectric loss, and the measuring frequency is becoming wider ranging from what is called wireless frequency to very high-frequency. A standard variable resistor of a high resistance value which is highly reliable throughout the wide frequency range is needed for the accurate measurement of a very small loss angle.

Recently synthetic insulating materials of excellent properties have been developed one after another and have been put into practical use. However instruments capable of accurately measuring a and tan 8 of an insulator having a Q value 10 to 10" over the frequency range between I kHz. and l,000 mI-Iz. have not been developed.

SUMMARY OF THE INVENTION An object of the present invention is to provide a standard variable resistor of a high resistance value for high frequency use which is highly reliable and has a high precision over a wide frequency range in order to meet the above-mentioned requirement.

The present invention brings such an advantage that a standard variable resistor of a high resistance value which has a high precision, a wide frequency range and a wide range of variable resistance value can be provided in a relatively simple construction. This desirable result is accomplished by use of a novel complex series condenser comprising three spaced, parallel plates wherein the intermediate plate is mechanically movable parallel to the outer plates. The plates comprise discs of dielectric materials having thin metal electrodes attached to their facing surfaces; and the electrode which is attached to one of said outer plates, and which faces said intermediate plate, is grounded. A fixed resistance is connected between said grounded electrode and the facing electrode on said intermediate plate. Measurements circuits are provided incorporating the described structure.

BRIEF DESCRIPTION OF THE DRAWINGS FIGs. 1A, 1B and 1C respectively illustrate a combination of parallel plate capacitors and a resistor, and represent the principle of the standard variable resistor of a high resistance value for high frequency use according to the present invention.

FIGS. 2A and 2B illustrate the equivalent circuits of the combination shown in FIG. 1B.

FIGS. 3A and 3B illustrate the equivalent circuits of the combination shown in FIG. 1C.

FIG. 4 illustrates a first embodiment of the present invention operative to measure an unknown resistor Rx using the standard variable resistor of a high resistance value for high frequency use.

FIG. 5A illustrates a second embodiment of the present invention in which the variable resistor of a high resistance value for highfrequency use according to the present invention is connected to a balanced tuning circuit for measuring purposes.

FIG. 58 illustrates the equivalent circuit of the variable resistor shown in FIG. 5A.

FIGS. 6A and 68 respectively illustrate a third embodiment of the present invention in which a variable resistor for highfrequency use is connected to a tuning circuit having a grounded common line, and the equivalent circuit thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention will hereinafter be described in conjunction with the drawings. In FIG. 1A, circular parallel plates, or disc I and 2, which respectively have the same area A, comprise dielectric members of a dielectric constant having respective thicknesses rs, and rs,. Discs I and 2 are disposed in spaced parallel relation to one another, and are parallel to a dielectric plate E, to provide gaps t, and t, on both sides of the disc 2. Very thin metal films or foils acting as electrodes are attached to the lower face of disc I, to both faces of disc 2, and to the upper face of plate E; and a metal terminal T, is disposed adjacent the upper face of disc 1. Elements 1 and 2 accordingly constitute first and second parallel plate capacitors respectively. A grounded terminal T, is disposed adjacent the lower face of plate E. A fluid dielectric substance (air or liquid) having a dielectric constant e fills the gaps t, and t,. For purposes of understanding the subsequent description, it is convenient to designate t,,'t +t,, and to designate ts=ts,ts,. In accordance with the present invention, and adopting these designations, ts For example, if t =l mm., then t =l0-20 mm. The intermediate disc capacitor 2 located between the disc capacitor 1 and the dielectric plate E is made movable, through a small displacement while being maintained in parallel relation to the disc capacitor 1 and to the plate E, by means of a mechanism such as a micrometer. Even though the intermediate disc capacitor can be moved, t remains constant.

The plate E is constructed as follows. A disc insulator of appropriate thickness is disposed between the aforementioned thin metal electrodes which are attached thereto. The lower one of these electrodes, attached to disc 2, has a thickness between 0.5 mm. and 1 mm. This lower electrode on disc 2 is connected, through a fixed resistor R, to the upper electrode on plate E (see Figs. 18 and 1C); and said upper electrode on plate E is in turn grounded by being connected to the aforementioned terminal T The capacitances which are constituted by the gaps t, and I, and the respective metal electrodes bounding said gaps, are designated c and C respectively. C is equal to e e,A/r,, where 20 is the permittivity of free space. The capacitor group shown in FIG. 1B is represented by the equivalent circuit shown in FIG. 2A, and the following equation holds.

Cr eoesA 6061A ueis where t,==t, +t

An angular frequency 1 equals 2m, where f is the frequency of the power source used. In FIG. 2A, if the expression wC- R 1O ...(2) is satisfied, the equivalent circuit of FIG. 2A may be transformed into that of FIG. 28. And the following expressions hold in FIG. 2B.

Then, as shown in FIG. IC, when the intermediate capacitor 2 makes a small displacement Ar toward the plate E, the

equivalent circuit will become as shown in FIG. 3A, where i 1tS+s(t1+At Q1A Cr T aisle /4 and C2 7,,- At

Further, the equivalent circuit of FIG. 3A may be transformed into that of FIG. 3B. In FIG. 3B, the expression.

Q); 1r|+ s( 1+ Rpx-(1+ Rs- 1+m) Rs From the above results, a standard variable resistor of a high resistance value for high-frequency use is materialized to be adjustable by changing Ar.

ln FIG. 4 which shows a first embodiment of the present invention wherein the standard variable resistor for highfrequency use above described is applied to a measuring circuit, at first, a test sample of a dielectric substance having a capacitance Cx and a resistance R1 is disconnected from the circuit by a switch S. In this state, the circuit is tuned by adjusting Cv. Let the tuned value of Cv be Cv and the current flowing through the detector to be lgo. Next, after connecting the test sample of dielectric substance (Cx, Rx) in parallel with the terminals T, and l: by closing the switch S, the circuit is again tuned by the adjustment of Cv. Then the following expression holds: Cx-=Cv,Cv where Cv is the tuned value of Cv when the sample is connected. In this case, the current flowing through the detector decreases from lgo due to the connection of Rx, but the said current can be made equal to Igo by adjusting the capacitor 2 of the variable resistor for high-frequency use to make a small displacement toward the plateE. Letting the displacement be At when the current flowing through the detector becomes [go after the sample is connected to the circuit, from the relation of:

the following expression holds.

1 Rpo/RE:

Substituting Rpo and Rpx in the expressions (3) and (4) for those in the equation (5), the following expression (6) is derived.

e, t, H 1

In the expression (6), the values of el, as, t 1,, and Rs are all known, so that the resistance value of the high value resistor Rx may be obtained from the measurement of At. Further, the Q-value of the test sample can also be obtained form the relation of QF-mCxRx.

ln a special case that the capacitor 2 is firstly set as t,,=z, and the fluid dielectric substance is air, hence 6, being equal to unity, the resistance value of the high value resistor Rx can be obtained from the following expression (7).

For example, if various parameters are chosen as follows, the diameter of the electrodes 40 mm., t =l mm. t,=t =0.5 mm. t,=l0 mm., Rs=40 KO, (for a glass plate), andf=2 l0 Hz., the relation of wCzR #i0 holds, and if At=l()p., Rx=36 MO is obtained form the expression (7). If a liquid of 510 is used as the fluid dielectric substance, Rr=l,764 MO is obtained if At=l0).t, so that it is understood that the resistance value of the high-value resistor Rx can be measured over a very wide range of resistance value.

The second and the third modified embodiments of the present invention, wherein the standard variable resistor for high-frequency use according to the present invention is applied to measurement circuits, are described with reference to FIGS. 5A and SB and FIGS. 6A and 68 respectively.

In F IG. 5A, there are shown parallel plate capacitors C, and C, having airgaps t, and 1, respectively. An intermediate metal plate is grounded and the capacitance values of the capacitors C, and C, are represented by C,=e,,A/t, and C,=e,A/t,, respectively, where A is the area of each of the electrode plates of the capacitors C, and C which are not grounded and e, is the permittivity of free space. The intermediate grounded plate is made movable to make a small displacement up and down by means of a micrometer. Now, assuming t,,=r,+t,, when the intermediate grounded electrode is slightly displaced up or down by a displacement At, the capacitance values of the capacitors C, and C, change respectively, but a capacitance value C across both end terminals A and B of the capacitors C, and C, is constant and never changes. That is,

In FIG. 5A, when a standard fixed value resistor for highfrequency use Rs (for example, a carbon film resistor) is connected in parallel with the capacitor C,, an equivalent circuit may be represented with C and Rx as shown in H6. 58, where the following expressions (8) and (9) of Rx and C will When the frequency f of a power source is designated by f the angular frequency w is expressed by w=21rf, and if the resistance value of the fixed resistor Rs is selected to satisfy the relation llwC Rs l or wC,Rs l, the expressions (8) and (9) may be respectively represented as follows by considering the relation w C Rs l.

by applying the relations C,=e,,A/t, and C =e A/t, to the above expressions, the following expressions are derived.

Rx=(t /t,) Rs l0) C,,,=s,,A/t,=constant (l l That is, the resistance value of Rx can be changed freely and continuously between Rs and infinity, while the capacitance value of C remains unchanged.

In order to obtain a very high resistance value for a variable resistor Rp, which is an equivalent presentation of the variable resistor according to the present invention as seen from terminals T, and T, when the value of r is not zero, capacitors Co which satisfy the relation C0 C are inserted in series with the capacitor C Then, Rp is represented by the following expression l2).

Thus, a standard variable resistor of a very high resistance value for high-frequency use is attained, whereby loss resistance of very low loss dielectric substances can be measured by a substitution method.

The test sample resistance Rx is represented by the following equation.

wherein r is the gap of the capacitor C, at the beginning of measurement and t is the gap thereof, and whereby a resistance value of the variable resistor according to the present invention can be substituted for Rxs of the text sample in the course of measurement.

The principle of the circuits in FIGS. 6A and 6B is completely the same as that of the circuits in FIGS. 5A and 58. It is modified only in that the intermediate movable plate between the parallel plate capacitors is insulated from ground and the capacitor C, has its lower electrode plate grounded, because the tuning circuit for measurement is grounded at one common line thereof.

lclaim:

l. A precision variable resistor for high-frequency use comprising three plates constituting a pair of outer plates and an intermediate plate mounted in spaced parallel relation to one another to define a pair of gaps therebetween each of said outer plates having a conductive electrode surface facing said intermediate plate, the opposing surfaces of said intermediate plate which face said outerplates respectively also being conductive, means for grounding the conductive electrode surface on one of said outer plates, a high frequency standard fixed resistor bridging the gap between said intermediate plate and said one of said outer plates, said resistor being connected between said grounded electrode surface on said one of said outer plates and the conductive surface of said intermediate plate which faces said grounded electrode surface, and means for moving said intermediate plate relative to said pair of outer plates while maintaining the parallel relationship between said three plates thereby to vary the relative dimensions of said pair of gaps.

2. The structure of claim 1 wherein said one of said outer plates comprises a dielectric plate provided with metal electrodes on both surfaces thereof, said metal electrodes being electrically connected to one another and to ground to form a grounded electrode;

the other one of said outer plates comprising a parallel-plate first capacitor including a dielectric member having a dielectric constant e, and a thickness I said intermediate plate comprising a thin parallel plate second capacitor including a dielectric member having a dielectric constant e, and a thickness t,, and having thin metal electrodes attached to both surfaces of said dielectric member.

said second capacitor being positioned in parallel with said first capacitor to define a gap t therebetween, and said second capacitor being positioned in parallel to said dielectric plate to define a gap r, therebetween, both of said gaps t, and I, being filled with a dielectric fluid having a dielectric constant 6 whereby said three plates and said pair of gaps comprise a combination of serially arranged capacitors:

said high-frequency standard fixed resistor having a resistance R, satisfying the condition: 2rrfC2R 10 where f designates a power source frequency, and C designates the capacitance of a capacitor constituted by the electrode of said second capacitor facing said gap t the grounded electrode of said dielectric plate, and the gap 1, filled with said dielectric fluid:

and means for connecting a test sample having an unknown resistance R,in parallel with said combination of serially arranged capacitors, whereby the magnitude of R, is measured by being substituted by a small displacement A! of said second capacitor based on the following relation:

3. A precision variable resistor resistor for high-frequency use comprising:

two serial parallel plate capacitors having capacitances C, and C, constituted by two fixed electrode plates and one movable electrode plate intervening between said two fixed electrode plates;

at least one capacitor having a small capacitance connected in series with said two serial parallel plate capacitors;

a high-frequency standard fixed resistor having a resistance R, connected in parallel with the one of said two serial parallel plate capacitors having a capacitance C,;

and means for varying said capacitance C, by shifting the position of said movable electrode plate relative to said two fixed electrode plates while satisfying the condition 211fC,R, l

where f designates a power source frequency, whereby the combination of said two serial parallel plate capacitors and said high-frequency standard fixed resistor is reduced to a parallel circuit of an equivalent fixed capacitance C and an equivalent variable resistance R x so that only the resultant resistance R I is freely and continuously varied while maintaining the resultant capacitance C at a constant value. 

1. A precision variable resistor for high-frequency use comprising three plates constituting a pair of outer plates and an intermediate plate mounted in spaced parallel relation to one another to define a pair of gaps therebetween each of said outer plates having a conductive electrode surface facing said intermediate plate, the opposing surfaces of said intermediate plate which face said outer plates respectively also being conductive, means for grounding the conductive electrode surface on one of said outer plates, a high frequency standard fixed resistor bridging the gap between said intermediate plate and said one of said outer plates, said resistor being connected between said grounded electrode surface on said one of said outer plates and the conductive surface of said intermediate plate which faces said grounded electrode surface, and means for moving said intermediate plate relative to said pair of outer plates while maintaining the parallel relationship between said three plates thereby to vary the relative dimensions of said pair of gaps.
 2. The structure of claim 1 wherein said one of said outer plates comprises a dielectric plate provided with metal electrodes on both surfaces thereof, said metal electrodes being electrically connected to one another and to ground to form a grounded electrode; the other one of said outer plates comprising a parallel-plate first capacitor including a dielectric member having a dielectric constant epsilon s and a thickness ts1; said intermediate plate comprising a thin parallel plate second capacitor including a dielectric member having a dielectric constant epsilon s and a thickness ts2 and having thin metal electrodes attached to both surfaces of said dielectric member. said second capacitor being positioned in parallel with said first capacitor to define a gap t1 therebetween, and said second capacitor being positioned in parallel to said dielectric plate to define a gap t2 therebetween, both of said gaps t1 and t2 being filled with a dielectric fluid having a dielectric constant epsilon 1, whereby said three plates and said pair of gaps comprise a combination of serially arranged capacitors: said high-frequency standard fixed resistor having a resistance Rs satisfying the condition: 2 pi fC2Rs 10 where f designates a power source frequency, and C2 designates the capacitance of a capacitor constituted by the electrode of said second capacitor facing said gap t2, the grounded electrode of said dielectric plate, and the gap t2 filled with said dielectric fluid: and means for connecting a test sample having an unknown resistance Rxin parallel with said combination of serially arranged capacitors, whereby the magnitude of Rx is measured by being substituted by a small displacement Delta t of said second capacitor based on the following relation:
 3. A precision variable resistor resistor for high-frequency use comprising: two serial parallel plate capacitors having capacitances C1 and C2 constituted by two fixed electrode plates and one movable electrode plate intervening between said two fixed electrode plates; at least one capacitor having a small capacitance connected in series with said two serial parallel plate capacitors; a high-freQuency standard fixed resistor having a resistance Rs connected in parallel with the one of said two serial parallel plate capacitors having a capacitance C2; and means for varying said capacitance C2 by shifting the position of said movable electrode plate relative to said two fixed electrode plates while satisfying the condition 2 pi fC2Rs>1 where f designates a power source frequency, whereby the combination of said two serial parallel plate capacitors and said high-frequency standard fixed resistor is reduced to a parallel circuit of an equivalent fixed capacitance CAB and an equivalent variable resistance RX so that only the resultant resistance Rx is freely and continuously varied while maintaining the resultant capacitance CAB at a constant value. 